this-> pointers

leogoldseed · October 14th, 2008, 12:12 AM

I am having trouble understanding a function that uses a ->this pointer. I also know that the function may work without using a ->this pointer, but the point is that I don't understand how the compiler understands it, or how it actually works, after reading it.
(Page 358) Ivor Horton's book:

// Ex7_10.cpp
// Using the pointer this

#include <iostream>
using std::cout;
using std::endl;

class CBox // Class definition at global scope
{
public:
CBox(double lv = 1.0, double bv = 1.0, double hv = 1.0) // Constructor definition
    {
    cout << endl << "Constructor called.";
    m_Length = lv; // Set values of
    m_Width = bv; // data members
    m_Height = hv;
    }

double Volume() // Function to calculate the volume of a box
    {
    return m_Length*m_Width*m_Height;
    }

int Compare(CBox xBox) // Function to compare two boxes which returns true (1) if the first is greater than the second, and false (0) otherwise
    {
    return this->Volume() > xBox.Volume();
    }

private:
    double m_Length; // Length of a box in inches
    double m_Width; // Width of a box in inches
    double m_Height; // Height of a box in inches
};

int main()
{
CBox match(2.2, 1.1, 0.5); // Declare match box
CBox cigar(8.0, 5.0,1.0); // Declare cigar box

if(cigar.Compare(match))
cout << endl << "match is smaller than cigar";
else
cout << endl << "match is equal to or larger than cigar";
cout << endl;

return 0;
}

So as you can see, this compare() function that was defined in the class definition returns 1 if the statement -this->Volume() > xBox.Volume()- is true, and 0 if not.

BUT WHY?? I don't get it. WHo decided is going to return a 1 or a 0? shouldn't it need an if (xxx.volume() > xxx.volume())
return 1;
else
return 0;

??

I really don't get it, and I don't want to just disregard it, because it still looks a little "magical" And I can't find any explanation as to why it's capable of doing this in the book.

Any help would be really appreciated. Thanks in advance!

cpppuneet · January 30th, 2009, 04:19 AM

this pointer points to object who is invoking a member function of a class,thus *this gives the object itself

regards
puneet vyas

sh_erfan · September 7th, 2010, 04:26 AM

look at this example:

Code:

//somewhere in the code

int a = 100, b = 99;
bool result;
result = a > b;

The above code initializes result to 1. Exactly,the same happens in your Compare function's return statement.
Remember that every comparison in your code is replaced by true or false (boolean values).After the expression this->Volume() > xBox.Volume() evaluated, the value true or false sits instead of the expression.And then the true/false is implicitly converted to an integer corresponding true/false (that is 1/0) when returned by the function.
So your function's return statement changes to one of these :

Code:

return 1;

or

Code:

return 0;

Hope this short post helped;